Recent work has shown the benefits of panoramic imaging, which is able to capture a large azimuth view with a significant elevation angle. If instead of providing a small conic section of a view, a camera could capture an entire half-sphere at once, several advantages could be realized. Specifically, if the entire environment is visible at the same time, it is not necessary to move the camera to fixate on an object of interest or to perform exploratory camera movements. This also means that it is not necessary to actively counteract the torques resulting from actuator motion. Processing global images of the environment is less likely to be affected by regions of the image that contain poor information. Generally, the wider the field of view, the more robust the image processing will be.
A panoramic camera is a device that captures light from all directions (i.e., 360 degrees), either as still images or as a continuous video stream. The images from such a device can be geometrically transformed to synthesize a conventional camera view in any direction. One method for constructing a panoramic camera combines a curved mirror and an imaging device, such as a still camera or video camera. The mirror gathers light from all directions and re-directs it to the camera. Both spherical and parabolic mirrors have been used in panoramic imaging systems.
Numerous examples of such systems have been described in the literature. For example, U.S. Pat. No. 6,118,474 by Nayar discloses a panoramic imaging system that uses a parabolic mirror and an orthographic lens for producing perspective images. U.S. Pat. No. 5,657,073 by Henley discloses a panoramic imaging system with distortion correction and selectable field of view using multiple cameras, image stitching, and a panflit-rotation-zoom controller.
Ollis, Herman, and Singh, “Analysis and Design of Panoramic Stereo Vision Using Equi-Angular Pixel Cameras”, CMU-RI-TR-99-04, Technical Report, Robotics Institute, Carnegie Mellon University, January 1999, discloses an improved equi-angular mirror that is specifically shaped to account for the perspective effect a camera lens adds when it is combined with such a mirror.
Affixing the mirror to the camera is problematic, since any support structure necessary must appear in the device's field of view. One approach is to make this support structure transparent, by using a glass cylinder that mounts onto a standard camera lens mount. Another approach is to use a center post to support the mirror.
Both of these approaches have drawbacks. Light from the sun or another bright source striking the glass cylinder can produce a “flare”, or line of bright illumination, in the panoramic image. Glass cylinders also attenuate the incoming light, leading to a darker image. The cylinders accumulate dirt, dust, and fingerprints, all of which degrade the image quality. The center post approach has drawbacks as well. It avoids the flares and fingerprints of the glass cylinder, but it leaves the curved mirror exposed to surface dirt. Furthermore, the center post support is inherently weak and prone to bending and optical misalignment. Finally, there is the issue of mounting it to the camera. There is no easy way to affix it to the camera other than boring through the camera's lens or affixing the post to a transparent lens attachment.
Rather than using a center post for support, one or more side struts can be used. Side struts provide better support and protection for the mirror, and they can be anchored to a standard threaded ring for easy attachment to the camera. Unfortunately, side struts obstruct the camera's field of view. The use of glass struts has been proposed to minimize this effect, but they can still lead to flares in the image.
Thus there is a need for a method for eliminating strut images from digital images.